Is it permissible to buy a commodity and resell it at a higher price, and is it necessary to inform the buyer of the first purchase price?

There is no harm in selling an item for more than its purchase price, as this is the basis of trade. The seller is not obligated to inform the buyer of the purchase price. What is incumbent upon him is to be truthful, not to lie, not to deceive, and not to betray. There are indeed Hadith (definition) that prohibit false oaths to promote goods.